In "Rogue One," Lucasfilm's first standalone movie set in the era of "A New Hope," the tech and visual teams working on the film were able to recreate not only villain Grand Moff Tarkin, since Peter Cushing has long since passed away, but a youthful Princess Leia for a crucial scene.

The young Leia cameo makes sense, because "Rogue One" leads into the first "Star Wars" and focuses on how the rebels got the plans for the Death Star. If you recall, Leia puts the plans in R2-D2 to try and get them to Jedi Obi-Wan Kenobi.

ABC News' "Nightline" was given a backstage pass into how this was made created by the employees of Industrial Light & Magic, the special effects company George Lucas founded in 1975.

John Knoll, chief creative officer and senior visual effects guru for Lucasfilm, said Leia was part of the original story for "Rogue One" and Tarkin came later. For those who have seen the movie, it's clear Tarkin and a 19-year-old Leia were brought back using advances in digital technology.

"It's a lot a pressure, yeah. I mean, it's digital humans, it's the hardest thing in computer graphics," Knoll told "Nightline." "Well, we look at human faces all day, every day. So people are very attuned to, you know, seeing anything that -- that looks off."

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Knoll said people were nervous about bringing these characters back in this form, but "because of the nature of the film, the way it sort of mates up right to 'Episode IV,' I thought it was important that it sort of end with that Leia moment -- where she's handed the plans and then we see where it goes after."

He continued, "For Tarkin, it's because he's such an important part of 'Episode IV' and the Death Star program. And given that this movie is, you know, about the threat of the Death Star, what it represents, and then how the rebellion responds to that threat. I thought it was important that Tarkin be part of this."

Tarkin was brought back, with the blessing of the Cushing estate, using the effects, but also the aid of a real motion capture actor, Guy Henry. The artists at ILM used his performance as a guide before putting a digital Tarkin over him.

Knoll said the reactions to the return of the "Star Wars" icons has been mostly positive.

"I've seen -- a lot of comments that went along the lines of, 'Aw, I had no idea [Tarkin] was computer generated -- until somebody else told me,'" he said.

As for Fisher herself, who died last week, Knoll said he got her blessing before entering her cameo in the film.

"She was involved in the process and, you know, she saw the final result and she loved it," he said. "She got to see the scene. [Kathleen Kennedy, president of Lucasfilm] showed it to her. So, I got a call afterwards from Kathy saying, 'Well, Carrie loved it.'"

How did they create a 19-year-old Fisher?

"I think it's a really good match to what she looked like in 'Episode IV.' We had really good scans of her and ... we matched a couple of frames very exactly with our CG model and then sort of bounced back and forth between our render and the archival frame," he added.

"Rogue One: A Star Wars Story" is in theaters now.

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